Fix the linkage errors caused by duplicate symbols

[adonis0147]

Recently, I met some linkage errors while using LLD linker to compile the source code of Apache Doris. The issue looks like #1809 .

src/butil/class_name.h

namespace detail {
template <typename T> struct ClassNameHelper { static std::string name; };
template <typename T> std::string ClassNameHelper<T>::name = demangle(typeid(T).name());
}

The linkage of ClassNameHelper<T>::name here is external due to it is variable template and it may cause the duplicate symbols error.

Some ways to fix it:

1. In C++ 17, use inline variable.
2. Before C++ 17, use unnamed namespace to make the linkage of ClassNameHelper<T>::name internal.

In this PR, I suggest using the second way to fix it.